The most common use of 아버님 어머님 is traditionally by married couples, especially the wife. The wife always calls her parents-in-law (i.e. the husband's parents) 아버님 and 어머님. The husband calls his wife's parents this way too, but he has another set of terms which is 장인 어른 (for father) and 장모님 (mother), so we can't say it is "always" as in the case of the wife. Before marriage (i.e. when they're 연인 사이), they'll use the same terms but they won't have very many occasions to do so as when they're married. This is because in the old days (until about 50 years ago), it was a standard practice for a married woman to live with her husband and his parents in the same house, likely using 아버님 and 어머님 several times a day to interact with them, but this is no longer the case now.
With all that said, 아버님 is not a word restricted to married couples or couples before marriage. It can be a general honorific term for 아버지 in other situations too. For example, a teacher might ask a pupil 아버님은 어떤 일을 하셔?, and so can you. I don't think what the other person told you is true.
By the way, the correct phrasing is 무슨 일을 하셔?, 하시니?, or 하시냐? (this last one sounds a little harsher than the others). 하셔니, 하셔냐 and 하시셔 are all wrong. 하셔니/하셔냐, which is a contraction of 하시어니/하시어냐, doesn't exist (and sounds very weird) since -어니/어냐 is not a proper verb ending form. 하시셔 is a contraction of 하시시어 which has a redundant second 시 in it (but many people mistakenly say it like this). It should be 하시어 or 하셔 with a single 시.